You'd have to establish a lot to turn the tables like that against existentialism, since the latter is grounded in science and much philosophy.
Hans Jonas argued that Gnosticism was proto-existentialist. In identifying the natural order or cosmos with the good, the ancient Greeks committed the naturalistic fallacy with abandon.
You seem to be talking about the historical Jesus, whereas the article was referring to the Jesus character from the gospels. Virtually nothing can be actually known about a historical Jesus.